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endorsing the conclusions of the Working Group on Relativity in Celestial Mechanics and Astronomy (RCMA),

Sub-Working Group of the Working Group on Astronomical Standards (WGAS)

The XXIIIrd General Assembly of the International Astronomical Union

considering that

- a relativistic solar system barycentric four-dimensional coordinate system with its coordinate time scale TCB was defined by International Astronomical Union (IAU) Resolution A4 (1991),

- a relativistic geocentric four-dimensional coordinate system with its coordinate time scale TCG was defined by IAU Resolution A4 (1991) and International Union of Geophysics and Geodesy (IUGG) Resolution 2 (1991), and

- the basic physical units of space-time in all coordinate systems were recommended by IAU Resolution A4 (1991) to be the SI second for proper time and the SI meter for proper length,

noting that

- practical realization of barycentric and geocentric coordinate systems in many groups (see International Earth Rotation Service (IERS) Standards, 1992) is based on time scales TDB and TT instead of TCB and TCG, respectively, and involves the scaling factors 1-LB and 1-LG for the spatial coordinates and mass factors GM in barycentric and geocentric systems, respectively, LB and LG being given in IAU Resolution A4 (1991),

- even more complicated scaling factors are introduced in the VLBI (Very Long Baseline Interferometry) model of IERS Conventions (1996), and

- astronomical constants and currently employed definitions of fundamental astronomy concepts are based on Newtonian mechanics with its absolute space and absolute time leading to ambiguities in dealing with relativistic effects,

recommends that

- the spatial coordinates of the Barycentric and Geocentric Reference Systems as defined by the IAU (1991) resolutions be used for celestial and terrestrial reference frames, respectively, without any scaling factors,

- the final practical realizations of the coordinate systems for use in astronomy and geodesy be implementations of the systems defined by IAU-IUGG (1991) resolutions,

- the use of TT for convenience of observational data analysis not be accompanied by scaling of the spatial geocentric coordinates,

- algorithms for astronomical constant determination and definitions of fundamental astronomy concepts be explicitly given within the basic reference systems envisaged by IAU-IUGG (1991) resolutions, and

- the IAU Working Group on Astronomical Standards (WGAS) continue the consideration of relativistic aspects of the concepts, algorithms and the constants of fundamental astronomy.

Created: 1 Jan 2001. Text provided by the former Central Bureau.

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